In addition to high strength and low weight, polyurethane Reaction Injection Molding (RIM) parts exhibit heat resistance, thermal insulation, dimensional stability, and a high level of dynamic properties. They also offer resistance to inorganic and organic acids as well as many other potentially damaging materials and chemicals including a large number of solvents. Resistance to weathering and aging is another plus, though extended exposure to the sun’s ultraviolet rays typically results in a color shift at the surface. Low processing temperatures (95F to 150F) and low injection pressures (30 to 100 psi) make the Reaction Injection Molding (RIM) process more economical than other molding methods for large parts.

In general it uses less energy to make the same product when compared to thermoplastics and requires less equipment and floor space. It’s also more automated than other thermoset molding alternatives. No energy consuming remelting of a solid thermoplastic is necessary, and the cycle times of the Reaction Injection Molding (RIM) process are competitive with thermoplastic injection molding of similar sized parts.

With this process, designers can exploit the encapsulation abilities of polyurethanes Reaction Injection Molding (RIM) systems to mold in-metal parts such as bushings, hinges, and frames. Many designers have used this capability to reduce or eliminate secondary manufacturing steps.

How does Reaction Injection Molding (RIM) Work?

Two liquid reactants—polyisocyanate component and resin mixture—are held in separate temperature controlled feed tanks equipped with agitators. From these tanks, the polyol and isocyanate are fed through supply lines to metering units that precisely meter the reactants under high pressure, to the mixhead. When injection begins and valves in the mixhead open, the liquid reactants enter a chamber in the mixhead at pressures between 1,500 and 3,000 psi where they are intensively mixed by high-velocity impingement. From the mix chamber, the liquid flows into the mold at approximately atmospheric pressure and undergoes an exothermic chemical reaction, forming the polyurethane polymer in the mold. Shot and cycle times vary, depending on the part size and the polyurethane system used. An average mold for an elastomeric part may be filled in one second or less and be ready for demolding in 30-60 seconds. Special extended geltime polyurethane RIM systems allow the processor to fill very large molds using equipment originally designed for molds with smaller volumes.